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 A354531 Numbers k such that 2*(2^k-1) is in A354525. 4
 1, 2, 3, 5, 7, 9, 13, 17, 19, 31, 61, 67, 89, 107, 127, 137 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Numbers k such that for every prime factor p of 2^k-1 we have gpf(2*(2^k-1)+p) = p. Numbers k such that for every prime factor p of 2^k-1, 2*(2^k-1)+p is p-smooth. All terms except 2 are odd: if k is even, then 3 is a factor of 2^k-1, so 3^m = 2*(2^k-1)+3 = 2^(k+1) + 1 => k+1 >= 3^(m-1). The only possible case is (k,m) = (2,2). Clearly A000043 is a subsequence. The exceptional terms (1, 9, 67, 137, ...) are listed in A354532. The next term is >= 349. The next composite term, if exists, is >= 7921 = 89^2. LINKS EXAMPLE See A354532. PROG (PARI) gpf(n) = vecmax(factor(n)[, 1]); ispsmooth(n, p, {lim=1<<256}) = if(n<=lim, n==1 || gpf(n)<=p, my(N=n/p^valuation(n, p)); forprime(q=2, p, N=N/q^valuation(N, q); if((N<=lim && isprime(N)) || N==1, return(N<=p))); 0); \\ check if n is p-smooth, using brute force if n is too large isA354531(n, {lim=256}, {p_lim=1<<32}) = { my(N=2^n-1); if(isprime(N), return(1)); if(n>lim, forprime(p=3, p_lim, if(N%p==0 && !ispsmooth(2*N+p, p), return(0)))); \\ first check if there is a prime factor p <= p_lim of 2^n-1 such that 2*(2^n-1)+p is not p-smooth (for large n) my(d=divisors(n)); for(i=1, #d, my(f=factor(2^d[i]-1)[, 1]); for(j=1, #f, if(!ispsmooth(2*N+f[j], f[j], 1<

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Last modified January 29 16:07 EST 2023. Contains 359923 sequences. (Running on oeis4.)